The present invention relates to a magnetic resonance imaging method and apparatus, and more particularly to a magnetic resonance imaging method and apparatus for acquiring data by a pulse sequence which generates a magnetic resonance signal for each TR (repetition time) and reconstructing an image based on the data.
In magnetic resonance imaging, spins within a subject are excited by an excitation pulse for each TR, and a magnetic resonance signal generated by the excitation is acquired as a gradient echo or a spin echo to fill a two-dimensional Fourier space. The magnetic resonance signal is given a different phase encoding for each view, and echo data are acquired for a plurality of views whose positions are different in the phase axis in the two-dimensional Fourier space. The echo data acquired for all views are then two-dimensional inverse Fourier-transformed to reconstruct an image.
In acquiring the echo data, a process is carried out involving: dividing the two-dimensional Fourier space into a central region containing the origin of the phase axis (at which the phase encoding amount is zero) and a range in close proximity to the origin in which the absolute value of the phase encoding amount is small, and peripheral regions on both sides of the central region in which the absolute value of the phase encoding amount is large; conducting data acquisitions so as to fill one peripheral region, the central region and the other peripheral region in order with view data, then conducting data acquisitions so as to fill the central region and the one peripheral region in order with view data, and then conducting data acquisitions so as to fill the central region and the other peripheral region in order with view data; and repeatedly conducting the data acquisition operations, thereby reducing the time interval between the data acquisitions for the central region relative to the time interval between the data acquisitions for each peripheral region.
When such data acquisitions are conducted, the image reconstruction is carried out using a set of data consisting of the data acquired for the central region and the data acquired for the peripheral regions on both sides of the central region. The data of the peripheral regions on both sides used here are the data whose acquisition time periods are nearest to the data acquisition time period of the central region. This causes a plurality of successive images corresponding to the successively updated data for the central region to be reconstructed.
Each reconstructed image indicates the state of the subject (time phase) substantially in a time period in which the data for the central region is acquired. Because the time interval between the data acquisitions for the central region is reduced by the above-described data acquisition process, the images successively,reconstructed indicate the time phases of the subject in detail.
According to the data acquisition process, however, since the data acquisitions for those regions are conducted in view number order in a sequential manner, the data acquisition time period of one of the peripheral regions on both sides of the central region is different from that of the other of the peripheral regions. Since these peripheral regions are given the phase encoding amounts having the same absolute values but opposite signs, any motion of the subject impairs symmetry of data as long as the time phase of the data for one peripheral region is different from that for the other peripheral region, and hence, may lead to artifacts in the reconstructed images.